1. Technical Field
This invention relates to crosspoint matrices and, more particularly, to a method and apparatus for scanning a matrix of switchable elements such as a keyboard.
2. Description of the Prior Art
Methods and apparatus for scanning a matrix of switchable elements, such as a keyboard, are generally known in the developing art of business machine and data terminal manufacture. Thus it is known in the art to employ a special purpose microprocessor to scan a keyboard and to provide signals indicative of the particular keys that are operated. However, such a special purpose microprocessor is limited in the number of leads on which data can be input and output. As a result, the size of the keyboard that the microprocessor is capable of scanning is correspondingly limited.
FIG. 1 shows one common arrangement that uses a special purpose microprocessor for scanning a keyboard. Microprocessor 101 provides a scanning port 102 comprising eight leads for scanning eight rows of an eight-by-eight row and columnar matrix 100 and ten return leads for retrieving data identifying a switched element. In particular, a data retrieval port 103 comprising eight leads provides information as to the column number of the switched element, and shift and control leads 104 and 105 provide information as to the shift or control status of the switched element. A separate output port 107, comprising eight leads, and a strobe lead 106 indicate to the processor 108 the particular code of the switched element. The keyboard scanning capacity of such special purpose microprocessors is limited to sixty-four switchable elements while twenty-seven input and output leads are employed for scanning and code reporting.
The operation of microprocessor 101 comprises the steps of scanning the matrix 100 of rows and columns one row at a time by changing the state of one scanning lead at a time. If one of the switchable elements has switched in a particular scanned row, a connection is completed to a columnar lead and a corresponding signal may be read on the data retrieval lead of port 103 associated with the columnar location of the switched element. The last steps are to perform a translation of the location of the switched element to a particular ASCII code and to report the code of the element to the processor 108 over port 107, the processor being strobed to read port 107 by a strobe signal on strobe lead 106. Because a close relationship exists between the scanning method and apparatus of FIG. 1, it is desirable that a solution to the problem of increasing scanning capacity be compatible with existing scanning methods.
At present, the only known way of increasing the scanning capacity beyond the capacity of a special purpose microprocessor is by the addition of a peripheral interface adapter (PIA) or similar device. Referring to FIG. 2, one arrangement employing a peripheral interface adapter 210 is shown. Responsive to signals on control lead or leads 213 and scanning port 202, two scanning ports 211 and 212 are provided for scanning matrix 200 of increased size. As in FIG. 1, data retrieval port 203 and control and shift leads 204 and 205 report data identifying the switched element and its status respectively to microprocessor 201. Processor 208, as in FIG. 1, is strobed when a code for the switched element may be read on output port 207. The switchable element capacity of matrix 200 is increased to one hundred twenty-eight. While the arrangement permits scanning a sixteen by eight matrix, it requires the addition of a peripheral interface adapter. The result is an increase in the cost and complexity of the key scanning circuit.
Thus it is clear from the above that a more compact and economical solution to the problem of expanding a keyboard's capacity is needed. It is desirable that there be no increase in the number of data input or output leads or ports of microprocessors employed for scanning and that the peripheral interface adapter circuit be eliminated.